Blowing in the Wind

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"Our primary reason for this study is to address one of the old chestnuts in coral reef science," says Hatcher. He explains that one of the ways in which atolls form is the result of a change in sea level. Atolls begin as fringing reefs surrounding a volcanic island (Darwin 1842). Through the process of global warming, glacial melting and/or island subsidence, the level of the sea gradually rises relative to the seabed and water begins to overtake the island. Since most reef-building corals cannot grow easily at depths of more than 150 feet (45 m) below the ocean’s surface, they will begin constructing their protective calcium carbonate encasements on top of one another at a rate fast enough to keep up with the sea level rise. At the same time the corals at the surface grow laterally to stay abreast of the ever-diminishing coastline. Provided that the sea does not rise too rapidly, the corals will continue to push upward and outward well after the volcanic island is completely submerged (Fagerstrom 1987).

The final shape that an atoll takes isn’t one of a giant tube ascending from the ocean floor. Rather, assorted detritus and dead coral will pile up on the inside of the coral ring and fill the void where the island used to be. From above, most atolls end up resembling an elliptical array of coral reefs with steep sides surrounding a relatively shallow lagoon in the center. If one could dive into the ocean and view an atoll from the side at a distance, it would resemble a very steep volcano with the topmost portion of the crater grazing the ocean’s surface (Huxley 1873).

Hatcher explains that these basic truths of coral atoll formation and structure have been accepted in one form or another for the past century. Yet, many of the finer points remain open for debate. One such dispute has arisen over the effect wind and wave activity in a region has on the width of the reefs that make up an atoll at sea level. "The dispute dates back to the middle of the nineteenth century and involves a disagreement between Charles Darwin and a James Dwight Dana [an American coral reef pioneer]," says Hatcher.

While atolls generally take the shape of the volcanic island they originally surrounded, the widths of the reefs that make up an atoll can vary quite a bit. Often times the reefs on one section of an atoll may be much wider than the rest, giving the atoll an irregular, oftentimes ocular shape when viewed from above. Charles Darwin believed that such differences in reef width were wholly due to the topography of the island’s coastline where the reef first took root. Gently sloping seafloors around an island would produce wider reefs and steep seafloors would produce thin reefs. Unless the atoll experienced significant erosion as it grew, he professed that the reefs would retain their original shape as they metamorphosed into an atoll. "With this vertical growth argument, the body of the atoll now apparent at the ocean’s surface is nothing but a direct reflection of the shape that was apparent when the current reefs started growing years before," says Hatcher.
 

 

Illustration of Island Evolution
The standard theory of atoll formation states that a volcanic island forms in deep tropical waters, giving coral polyps a foundation to grow on (above, top). In time, the volcano becomes dormant and the island begins to subside. Coral reefs, originally fringing the edges of the island, become a barrier reef outlining the contour of the original coastline (above, middle). After the original island slips entirely beneath the waves, all that is left is a coral atoll (above, bottom). (Images by Robert Simmon, NASA GSFC)

The site Oceanography from the Space Shuttle has further examples of the evolution of coral reefs.)

 

Dana put forth another theory ten years or so after Darwin. He argued that the current shape of an atoll had more to do with the patterns of wind and wave activity. Specifically, more rapid movement of waters around the outer edges of a living reef provides the reef-building coral polyps and their symbiotic algae (zooxanthellae) with more of the calcium, nutrients, and food particles they need to grow. The net effect is that the reefs become wider and denser over time near the locations where the water flow is the greatest. "So, in essence, this horizontal growth argument goes that the density and thickness of a reef are determined by the amount of nutrients and minerals delivered by the waters flowing around it," says Hatcher.

Despite all the research on corals that has taken place since Darwin and Dana’s time, the debate has never been put to rest. The problem hasn’t been one of intelligence or of effort on the part of the scientists, but of logistics. In order to settle this issue, a researcher would need evidence of many atolls’ interactions with the waves and currents over a long period of time. Since atolls grow upwards at a maximum rate of only 1.2 centimeters per year, witnessing the process first hand is out of the question (Nature Conservancy 2000). Attempting to discern the development of the reefs from current clues can be very difficult. Most atolls simply are not mapped to the level of precision needed to understand the nuances of how they formed. In the few instances where atolls have been mapped, the effort was both pricey and time consuming.
 

 


Another factor influencing atoll growth is the flow of water around the reefs. For example, prevailing winds can agitate water on one side of an island, stimulating coral growth. (Red arrows in the image above indicate the prevailing wind direction.) On the lee side of the island the coral do not receive as many nutrients, resulting in slower growth and a thinner reef. (Image by Robert Simmon, NASA GSFC)

 

Hatcher and Naseer became intrigued by this debate while observing the health of the coral reefs of the Maldives. These islands are made up of a cluster of 22 atolls grouped in an elongated formation stretching across roughly 823 kilometers (511 miles) north-south in the Indian Ocean south of Sri Lanka. Each of the atolls is made up of several dozen reefs. Rubble and sediments derived mainly from dead coral have piled up on some of these reefs to form low, flat islands where roughly a quarter of a million people now live.

Here the researchers saw a pattern that seemed to confirm Dana’s theory on coral reef formation. "There is a strong consistency in the asymmetry of the reefs that form the atolls in the Maldives," says Naseer. Specifically, the reefs that face the open ocean to the west and to the east tend to be a little wider and denser than those reefs that face the atoll lagoons or those that are protected in the inner lagoons between atolls. These are the same directions from which the characteristic monsoons blow. In the winter they come in from the east and then for the remaining three seasons they blow even harder from the west. The monsoons bring larger waves and stronger currents to the margins of the unshielded reefs and stir up cooler, nutrient-rich waters.

next Analyzing Atolls
next Mapping the Maldives’ Amazing Atolls

 

Section 
of a Map of the Maldives
The Maldives are a particularly complex archipelago. Individual atolls may be composed of many islands and reefs, and islets and reefs often fill the atolls' interior. (Map courtesy Abdula Naseer)

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